Liquid crystal display devices utilize optical (refractive) anisotropy and dielectric anisotropy of liquid crystal materials. Liquid crystal display devices have widely been used for the display of watches, word processors, computer terminals, and televisions. Liquid crystal materials mean principally liquid crystal compounds exhibiting a liquid crystal phase. These materials have been used in a form of a composition in which various kind of compounds are usually mixed. At that time when the composition is produced, compounds which do not exhibit by themselves a liquid crystal phase, but can exhibit a liquid crystal property when mixed with a liquid crystal compound or liquid crystal composition and thus clearly have a latent liquid crystallinity are also useful. For purpose of the present invention, the term "liquid crystalline compounds" include the latter compounds having a latent liquid crystallinity in addition to the liquid crystal compounds.
While various properties are required of liquid crystal compositions, optical anisotropy (.DELTA.n) is one of especially important properties. Since when the product (.DELTA.n.multidot.d) of the .DELTA.n and cell thickness (d) is a specific value (first minimum or second minimum), display qualities (contrast and viewing angle) of liquid crystal display devices become optimum, cells now being practically used have been designed according to the optimum value. However, in order to satisfy other required performances up to maximum, several different values are sometimes selected for d under the conditions described above. For instance, increase of response speed is strongly required lately, and there is a trend toward a small d with an improvement in cell preparation technology for a background to satisfy the requirement. Accordingly, exploitation of liquid crystal compositions having a high optical anisotropy, namely, exploitation of liquid crystalline compounds which can achieve a high optical anisotropy have been demanded.
Heretofore, compounds expressed by any one of the formulas (10) to (12) are disclosed, as liquid crystal compounds having a comparatively high .DELTA.n, in Laid-open Japanese Patent Publication No. Sho 61-5031, Mol. Cryst. Liq. Cryst., 48, 175 (1978), or Laid-open Japanese Patent Publication No. Hei 2-180,840. ##STR2##
However, compounds expressed by the formula (10) do not exhibit a sufficiently high .DELTA.n. Whereas compounds expressed by the formula (12) exhibit a comparatively high .DELTA.n, the compounds are considerably poor in miscibility with other liquid crystalline compounds. Whereas compounds expressed by the formula (11) exhibit a comparatively high .DELTA.n and good miscibility, the compounds are considerably poor in chemical stability and have such a problem that they are naturally decomposed even when allowed to stand at room temperature.